Surface and catalytic properties of Al 2O 3–ZrO 2 solid solutions prepared by sol–gel methods

Abstract

A series of ZrO 2–Al 2O 3 solid solutions was prepared by co-hydrolysis of Al-tri-sec-butoxide and Zr-propoxide, having molar ratios X=Zr/(Zr+Al)=0.0.29, 0.53, 0.63, 0.71, 0.81, 1.0. A systematic study was peformed by X-ray diffraction, BET-N 2 adsorption, electron microscopy, potentiometry (butylamine) and NH 3-TPD. The as synthesized ZrO 2 included monoclinic, tetragonal and cubic phases, i.e. XRD peaks at (2Θ)=28.2° and 31.5° for the m-phase, (2Θ)=30.2° and 35.0° for the t-phase and (2Θ)=50.5° and 59.5° for the c-phase. The progressive formation of a solid solution occurs, between Al 2O 3 and ZrO 2 along the composition axis, with poorly crystallized mixed phases. The BET-N 2 isotherms from X=0.2–1.0 showed a Type IV (BDDT) profile, typical of mesopores in the interval between 20< D p<500 Å, the S BET varied from 200 to 250 m 2 g −1 with ( S BET) MAX=246 m 2 g −1 for X=0.2. The latter composition had a mean pore diameter ( D p) equal to 250 Å, but for X=0.5 the value increased to D p=700 Å. For X=0.5 and 0.6 there is a bimodal distribution centered at 25 and between 200 and 700 Å, respectively. For X=0.8 and 1.0 the interval moves from 200 to about 1000 Å. The acid sites of the series Al 2O 3–ZrO 2 were titrated with n-butylamine and NH 3, showing that higher acid sites densities (DAS) are obtained for X=0.20 and 0.50, i.e. 10.8 μmol Bu m −2, but ZrO 2-rich solids with X=0.6 and 0.8 showed DAS values minor than 10%, while single ZrO 2 phase was 50% more acidic, i.e. 15 μmol Bu m −2. For NH 3 the highest DAS values were for ZrO 2 and Al 2O 3, then for X=0.2 and 0.8 the total amount of NH 3 adsorbed was 1.78 and 1.8 μmol (NH 3) g −1, respectively. The reaction of 2-propanol at 453 K on the Al 2O 3–ZrO 2 series showed for X=0.20, i.e. a conversion of 4 μmol h −1 m −2, against 2.32 and 1.16 for the single Al 2O 3 and ZrO 2 phases, respectively. The selectivity towards C 3 = at 453 K ( S p) was superior for the extremes having X=0.20 and 0.80, i.e. 87 and 100%, respectively, while for acetone ( S A) there is a maximum at X=0.6 and for di-isopropyl-ether ( S E) the alumina ( X=0) showed the highest selectivity, i.e. 18%. At higher temperatures (i.e. 513 K) the selectivity towards C 3 = was predominant, i.e. 91–98%.